In a democratic society our principles of justice must be based on the idea that all citizens are free and equal moral persons because basic equality is a necessary condition for democratic citizenship. Thus, only what citizens would rationally will from a shared point of view can be just because it is the only mechanism that respects citizens as free and equal.

Samuel Freeman [Avalon Professor in the Humanities at The University of Pennsylvania. Justice and the Social Contract: Essays on Rawlsian Political Philosophy. Oxford University Press. New York, NY. 2007. 40-42]

[Kantian constructivism] is a more specialized notion that works within this constructive model of justification; it explains why we may find some of our considered convictions recalcitrant. It aims to show that the principles that best cohere with our convictions are not questionable accidents of culture but are objective in that they have a basis in our capacities for practical reasoning. These capacities, Rawls presumes, underlie many of our considered convictions, including our political conception of ourselves as free and equal (JF, 233; TJ, sec. 77). Rawls seeks to capture this self-conception with an idealization, the [model conception] of free and equal moral persons. Toconstruct, in our first sense, principles of justice appropriate for us, Rawls [we] constructsasocialworldmodeledontheself-conception and practical capacities offreeandequalmoralpersons. The purpose of this second kind of construction is to give content to Kant's enigmatic [the] idea of autonomy as reason legislating principles for itself. In the absence of some kind of procedure that shows the relationship between our capacities for reasoning and moral principles, such phrases [this is] are difficult to make sense of. Agreement from the original position serves this role; it is a [procedural interpretation] of practical reason in matters of justice (CP, 345–46; TJ, 256/226 rev.) or, more exactly, of a conception of persons as both reasonable and rational.Since this procedure is designed to [model] the moral powers,51 the content of the principles chosen from that point of view will be determined by these reasoning capacities and the conception of the person to which they give rise (CP, 303, 306).52 In this sense, moral principles are [constructed] on the basis of reason.¶ The objectivity that Rawls ascribes to his [of these]principles rests on his claim that they would be willed and agreed to from a shared point of view, which is objective in that everyone abstracts from their particular (subjective) aims, beliefs, and perspectives to view society on an equal footing (TJ, 516–19/452–55 rev.). This conception of objectivity is practical, as opposed to theoretical, in the following sense: Rawls's claim is not that, being impartially situated, we all have a clear, undistorted view that allows us to make true judgments about a prior and independent moral order. In constructivism, Rawls says, there is no order of moral facts, prior to human reasoning, for our moral judgments to be true of (CP, 354). This does not mean that Rawls [one]must deny that a prior order of moral facts or principles can exist [rather], for that metaphysical commitment would conflict with his [the] practical aim. Rather, it means that if there is such an order, it is not because certain principles are true of it that we are bound, as democratic citizens, to follow them. What commits us, as citizens, to these principles is that they are [most reasonable for us] (CP, 340), in that they best accord with our capacities for practical reasoning in the circumstances of a democratic society. The conception of objectivity that informs this claim is practical, since the shared point of view from which we would agree to these principles is designed not to give us privileged access to a prior moral order but to represent our powers of practical reasoning in a way appropriate to our democratic conception of ourselves as free and equal.53

The shared point-of-view necessary for rational agreement is represented by the original position, the idea that citizens can imagine themselves behind a veil of ignorance. Behind the veil citizens would not know whether they were rich or poor, their gender, their race, or any other contingent fact about their place in society, and so would be represented purely as free and equal. And, that also means it is the only system that has no risk of bias and so no arbitrary decision-making. Preventing arbitrariness is a prerequisite to any framework because morality serves to guide action so it must prevent arbitrariness that contradicts rationally adopted rules and undermines any moral code.

Behind the veil, citizens would design a system that benefitted the least advantaged because 1) diminishing marginal utility dictates that helping the most advantaged provides less utility than the least advantaged. This is why giving a dollar to a homeless person would probably make them happier than giving a dollar to a billionaire; already having goods makes each additional good less valuable. 2) People are risk-averse, meaning that they would avoid risks of occupying the worst social position, but this would ensure reasonable treatment regardless.

Therefore, in order to rectify the arbitrary disadvantages of the natural lottery, we must adopt a principle of equality of opportunity.

The basic motivation behind the equality of opportunity (EO) principle is this: if you can walk into a nursery and correctly make statistical predictions like “this baby because of his skin color and/or his family’s socioeconomic status is likely to make $20,000 per year less when he grows up than that baby over there,” then the society in which such predictions can be made is unjust and ought to be changed so that such predictions are no longer true of it. [Equality of opportunity] attempts to mitigate[s] the effects of social caste. It requires a legal ban on racist/sexist hiring and promotion practices in the private sector, as well as state efforts to improve the educational opportunities of the poor by providing free public education, Head Start programs, etc. [Equality of opportunity] [and] does not forbid inequalities in wealth and power that are due to people's free choices. Suppose that I’m poor because I've chosen not to make the sacrifices necessary to develop highly marketable labor skills (I dropped out of the public high school because it bored me), or because I frittered away my inheritance from my grandmother playing Internet blackjack. You, on the other hand, have a large stock portfolio and a high paying job because you've saved and made sacrifices. EO as usually understood implies that there is no injustice in the inequality between us. I deserve my poverty and you your wealth, because they are due entirely to our choices. It is only some inequalities of outcome that [equality of opportunity] condemns, namely, the undeserved ones that are due to unequal opportunity.

Thus, the standard is ensuring equality of opportunity. Prefer the standard for 2 additional reasons:

First, people are moral equals unless a morally relevant distinction exists because no characteristics exist to differentiate them, so the results of the natural lottery shouldn’t arbitrarily disadvantage some people over others, which requires equality of opportunity.

Second, people born with disadvantages are subject to external pressures that affect the choices they have, which destroys their autonomy because they don’t have free reign over their own lives, which outweighs any individual violation on magnitude. And, autonomy is a prerequisite to moral theorizing because free will is responsible for the judgment of principles and commitment to a certain action.